build specific versions of packages with gub

[Graham Percival]

Since git master is still borked, I thought I'd make a stab at 2.12.  When I do:
    make LILYPOND_BRANCH=stable/2.12 lilypond

it builds the binaries (apparently) ok[1], but then it goes on to execute:
    python bin/gub --platform=linux-x86 --offline     lilypond-test

which attempts to build the tests (and later, docs) from master.

Now, bin/gib has a --branch option, but I can't see anything similar
for bin/gub.  Any thoughts?

Cheers,
- Graham

[1]  you can test the 2.12.3 packages if you want:
http://lilypond.org/~graham/